1. Field of the Invention
The present invention relates to light guide plates and back light systems using the light guide plates, and especially to such light guide plates and back light systems used in liquid crystal displays (LCDs).
2. Description of the Prior Art
Most portable electronic devices such as laptop and notebook computers, mobile phones and game devices have viewing screens unlike the cathode-ray-tube (CRT) monitors of conventional desktop computers. Users expect the viewing screens to provide performance equal to that of CRT monitors. To meet this need, computer manufacturers have sought to build flat panel displays (FPDs) offering superior resolution, color and contrast, while at the same time requiring minimal power consumption. LCDs are one type of FPD which satisfy these expectations. However, the liquid crystals of an LCD are not self-luminescent. Rather, the LCD needs a back light system which offers sufficient luminance (brightness) in a wide variety of ambient light environments.
A light guide plate is a key component of the back light system used in an LCD. Typically, the light guide plate has either of two shapes: a sheet having a uniform thickness (“planar”), or a wedge-shaped sheet (“wedgy”). Both these light guide plates convert a point light source or a linear light source into a surface light source. Generally, the planar light guide plate needs a plurality of pattern-dots on a bottom surface thereof, in order to disable partially total internal reflection of light beams and to provide more uniform brightness. The pattern-dots need to be accurately designed, which usually requires numerous optical simulations. Further, the accurately designed pattern-dots are difficult to produce. Unlike the planar light guide plate, the wedgy light guide plate has a bottom surface oblique to a light input surface. The bottom surface can eliminate total internal reflection of at least some of the light beams traveling from the input surface, without the need for pattern-dots. If pattern-dots are provided on the bottom surface of the wedgy light guide plate, they are an “optional extra” to enhance uniform brightness and the elimination of total internal reflection. Therefore the pattern-dots do not need to be very accurately designed, and they are relatively easy to produce.
As shown in FIG. 5, a conventional back light system 1 comprises a light source 11 and a wedgy light guide plate 10. The light source 11 is a cold cathode fluorescent lamp (CCFL). The light guide plate 10 comprises a light output surface 103, a bottom surface 102 opposite to the output surface 103, and a light incident surface 101 and three side surfaces (not labeled) interconnecting the output surface 103 and the bottom surface 102. A plurality of pattern-dots (not shown) is provided on the bottom surface 102.
In operation, light beams emitted from the light source 11 enter the light guide plate 10. Some of the light beams are reflected and then exit through the output surface 103, and other light beams directly exit through the output surface 103. As shown, relatively few light beams can reach an area of the output surface 103 that is far away from the light incident surface 101. Optical energy at an area of the output surface 103 that is most distal from the incident surface 101 is significantly reduced. Thus, even with the pattern-dots on the bottom surface 102, the back light system 1 has the disadvantage of uneven brightness. Furthermore, the back light system 1 has only one CCFL providing optical beams. The optical beams undergo partial loss when passing through other optical elements (not shown) of an associated LCD. Accordingly, the back light system 1 yields relatively low brightness for the LCD.